std::equal_range is used to find the subrange within a given range [first, last) that has all the elements equivalent to a given value. It returns the initial and the final bound of such a subrange.
This function requires the range to be either sorted or partitioned according to some condition such that all the elements for which the condition evaluates to true are to the left of the given value and rest all are to its right.
It can be used in two ways as shown below:

Comparing elements using <:
Syntax:
Template pair equal_range (ForwardIterator first, ForwardIterator last, const T& val); first: Forward iterator to the first element in the range. last: Forward iterator to the last element in the range. val: Value of the subrange to search for in the range. Return Value: It returns a pair object, whose member pair::first is an iterator to the lower bound of the subrange of equivalent values, and pair::second its upper bound. If there is no element equivalent to val, then both first and second points to the nearest element greater than val, or if val is greater than any other value, then both of them point to last.
// C++ program to demonstrate the use of std::equal_range
#include <iostream>
#include <vector>
#include <algorithm>
using
namespace
std;
int
main()
{
vector<
int
> v = { 10, 10, 30, 30, 30, 100, 10,
300, 300, 70, 70, 80 };
// Declaring an iterator to store the
// return value of std::equal_range
std::pair<std::vector<
int
>::iterator,
std::vector<
int
>::iterator> ip;
// Sorting the vector v
sort(v.begin(), v.end());
// v becomes 10 10 10 30 30 30 70 70 80 100 300 300
// Using std::equal_range and comparing the elements
// with 30
ip = std::equal_range(v.begin(), v.begin() + 12, 30);
// Displaying the subrange bounds
cout <<
"30 is present in the sorted vector from index "
<< (ip.first  v.begin()) <<
" till "
<< (ip.second  v.begin());
return
0;
}
chevron_rightfilter_noneOutput:
30 is present in the sorted vector from index 3 till 6
Explanation: After sorting the vector v1, we checked for the bounds within which 30 is present, i.e., from index 3 till index 6.
 By comparing using a predefined function:
Syntax:
pair equal_range (ForwardIterator first, ForwardIterator last, const T& val, Compare comp); Here, first, last and val are the same as previous case. comp: Binary function that accepts two arguments of the type pointed by ForwardIterator (and of type T), and returns a value convertible to bool. The value returned indicates whether the first argument is considered to go before the second. The function shall not modify any of its arguments. This can either be a function pointer or a function object. Return Value: It returns a pair object, whose member pair::first is an iterator to the lower bound of the subrange of equivalent values, and pair::second its upper bound. If there is no element equivalent to val, then both first and second point to the nearest element greater than val, or if val is greater than any other value, then both of them point to last.
// C++ program to demonstrate the use of std::equal_range
#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <functional>
using
namespace
std;
// Defining the BinaryFunction
bool
comp(
int
a,
int
b)
{
return
(a > b);
}
int
main()
{
vector<
int
> v = { 10, 10, 30, 30, 30, 100, 10,
300, 300, 70, 70, 80 };
// Declaring an iterator to store the
// return value of std::equal_range
std::pair<std::vector<
int
>::iterator,
std::vector<
int
>::iterator> ip;
// Sorting the vector v in descending order
sort(v.begin(), v.end(), greater<
int
>());
// v becomes 300 300 100 80 70 70 30 30 30 10 10 10
// Using std::equal_range and comparing the elements
// with 10
ip = std::equal_range(v.begin(), v.begin() + 12, 10, comp);
// Displaying the subrange bounds
cout <<
"10 is present in the sorted vector from index "
<< (ip.first  v.begin()) <<
" till "
<< (ip.second  v.begin());
return
0;
}
chevron_rightfilter_noneOutput:
10 is present in the sorted vector from index 9 till 12
Where can it be used ?

std::lower_bound and std::upper_bound at one place: This function can be used if we want to use both std::lower_bound and std::upper_bound at the same time, as its first pointer will be same as std::lower_bound and its second pointer will be same as std::upper_bound. So, there is no use of separately using them, if we have std::equal_range.
// C++ program to demonstrate the use of std::equal_range
#include <iostream>
#include <vector>
#include <algorithm>
using
namespace
std;
int
main()
{
vector<
int
> v = { 1, 2, 3, 4, 5, 5, 6, 7 };
// Declaring an iterator to store the
// return value of std::equal_range
std::pair<std::vector<
int
>::iterator,
std::vector<
int
>::iterator> ip;
// Using std::equal_range and comparing the elements
// with 5
ip = std::equal_range(v.begin(), v.end(), 5);
// Displaying the subrange bounds
cout <<
"std::lower_bound should be equal to "
<< (ip.first  v.begin()) <<
" and std::upper_bound "
<<
"should be equal to "
<< (ip.second  v.begin());
vector<
int
>::iterator i1, i2;
// Using std::lower_bound
i1 = std::lower_bound(v.begin(), v.end(), 5);
cout <<
"\nstd::lower_bound is = "
<< (i1  v.begin());
// Using std::upper_bound
i2 = std::upper_bound(v.begin(), v.end(), 5);
cout <<
"\nstd::upper_bound is = "
<< (i2  v.begin());
return
0;
}
chevron_rightfilter_noneOutput:
std::lower_bound should be equal to 4 and std::upper_bound should be equal to 6 std::lower_bound is = 4 std::upper_bound is = 6
This article is contributed by Mrigendra Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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